High frequency tuned circuits



Aug. 27, 1940. T LETT 2,212,839

HIGH FREQUENCY TUNED CIRCUITS Filed July 2, 1938 lA/VE/VTOR FEEDER/C 70MLET? BY ATTORNEY Patented Aug. 27, 1940 UNHE S signor to Electric &

Musical industries Limited, Hayes, Middlesex, England, a company ofGreat Britain Application July 2, 1938, Serial No. 217,124 In GreatBritain July 14, 1937 7 Claims.

This invention relates to high frequency tuned c rcuits and hasparticular reference to the tuned clrcuits of modulated carrier wavereceivers, the frequency band width of which it is desired to vary.

It is well known that damping is imposed on a tuned circuit connected inthe grid circuit of a valve when that valve has a largely capacitativeload in its anode circuit. The damping, known as the Miller efiect, is afunction of the magnification factor of the valve and also of its gridto anode capacity.

The object of the present invention is to provide an inexpensive buteffective means of varying the selectivity of a tuned circuit in, forexample, a wireless receiver provided with a screened grid valve orvalves.

According to the present invention in a circuit arrangement including avalve of the screened grid type having a tuned input circuit, in orderto vary the selectivity of said tuned input circuit, a condenserconnected to the screening grid of the valve is arranged to be connecteddirectly to earth or disconnected therefrom or to be connected to earththrough a resistance which may be variable. The condenser may constitutethe usual de-coupling condenser and the valve may be a tun d radio orintermediate frequency amplifier or a detector valve, a. switch beingprovided for effecting the changes in the connections to the condenser.

In order that the invention may be more clearly understood and readilycarried into effect, a portion of a circuit arrangement embodying theinvention will now be described more fully by way of example withreference to the accompanying drawing.

Referring to the drawing a tuned circuit 3 which may be the outputcircuit of a radio or intermediate frequency amplifier is coupled to thetuned input circuit 2 of a screened grid detector valve 3 provided witha grid leak resistance and condenser E, 5. The anode t of the detectorvalve 45 is coupled, for example through a resistance and shuntcondenser E, 8, to a low frequency output circuit not shown and thescreening grid 9 of the valve is connected through a resistance it to asource of high tension voltage. A de-coupling condenser H is connectedbetween the screening grid 9 and the cathode i2 through a variableresistance 13 across which is connected a switch M which enables theresistance to be short-circuited. This switch may, if desired, bearranged to leave the condenser open circuited.

I It will be seen that the arrangement provides for variation of theselectivity of the tuned circuit am a simple manner by operation of theswitch it to connect the de-coupling condenser ll directly to thecathode IE or through the resistance it. The operating conditions, whenthe switch it is open and the resistance 53 effective, will be modifiedby the different effective anode impedance which obtains when the switchit is closed and the resistance i3 is short circuited. The diiierentanode impedance will produce different damping effects upon the tunedcircuit 2 with consequent differences in thebreadth of tuning.Adjustment of the value of the resistance l3 provides a finer degree ofvariation oi selectivity. It is possible with the arrangement describedto vary the damping applied to the tuned circuit 2 from 10,000 to250,000 ohms due to the possible variations in the damping effect of thevalve.

I claim:

l. A circuit arrangement comprising in combination a thermionic valvehaving a cathode, an anode, a control grid and a screening grid, a tunedinput circuit connected between said cathode and said control and meansfor varying the selectivity of said tuned input circuit, said meanscomprising a condenser connected to the screening grid of said valve,and means for selectively connecting said condenser directly or throughan impedance to the cathode of said valve.

' A. circuit arrangement comprising in combination a thermionic valvehaving a cathode, an anode, a control grid and a screening grid, 2.tuned input circuit connecting between said cathode and said controlgrid and means for varying the selectivity of said tuned input circuit,said means comprising a condenser and a resistance connected in seriesbetween the screening grid and cathode, and means for selectivelyshort-circuiting said resistance.

3. A circuit arrangement comprising in combination a thermionic valvehaving a cathode, an anode, a control grid and a screening grid, 'atuned input circuit connected between said cathode and said control gridand means for varying the selectivityof said tuned input circuit, saidmeans comprising a condenser, and a variable resistance connected inseries between the screening grid and cathode, and means for selectivelyshort-circuiting said resistance.

4. A circuit arrangement according to claim 1 wherein said valveconstitutes an amplifier in a wireless receiver, said condenser being adecoua wireless receiver, said condenser being a decoupling condenserfor the screen grid of said valve and the selective short-circuitingmeans comprises a switch.

7. A circuit arrangement according to claim 2 wherein said valveconstitutes a detector valve in a wireless receiver, said condenserbeing a decoupling condenser for the screening grid of said valve andthe selective short-circuiting means comprises a switch.

FREDERIC TOM LETT.

